Gross anatomic, histologic and ultrastructural studies were made of the changes that developed in right ventricular-pulmonary artery conduits implanted in young baboons for 37 to 61 weeks. These conduits contained either a porcine aortic valve or a bovine pericardial valve. The major changes observed consisted of conduit obstruction by fibrous peel growing on its luminal surface. This fibrous peel tended to involve the bioprosthetic valve, causing cuspal retraction and interfering with valve function, such that many conduits behaved as valveless conduits.